This present invention concerns a method for the treatment of a noisy temporal digital signal yk corresponding to an initial analogue signal st after it has been conditioned by a conditioning chain, with the said initial analogue signal st being representative of information concerning radiation coming from a radiation source, where this radiation can exhibit an energy distribution.
Such a method, or a system designed to implement this method, is generally used in the context of detection, counting and measurement of events, such as detection of the said emitted radiation.
In particular its purpose is the construction of a spectrum that is used for measurement and analysis of the radiation supplied by the source.
It will be observed that, according to the invention, here radiation is considered to be any radiation that is designed to interact with a means of detection so as to be in possession of a usable temporal signal.
In this regard, and by way of non-limiting examples, the radiation covered by the invention in particular concerns photons, especially X and gamma radiation, nuclear particles, or even more generally, any particle or any packet of particles.
Thus a system or method of the type proposed above can in particular have as its purpose the construction of a spectrum that has a number of particles detected at a given energy, as a function of the energy.
This is interesting in particular in the case of particle sources exhibiting an energy distribution.
In fact it is known that a spectrum obtained from such a source can include spectral energy lines that are characteristic to it.
As a consequence, from a measurement of the particle source, it is possible to obtain a spectrum whose examination by a specialist or by software allows one to access information on the said particle source.
In particular, the examination allows one to identify the nature of the source studied.
In the non-limiting example of the gamma-ray field, a system or method of the aforementioned type can supply a spectrum of lines, such as that represented in FIG. 1, which allows us to identify the radio-elements that make up this source, and therefore to characterise the latter.
It will be observed that, by way of an example, FIG. 1 represents a normalised energy spectrum for Caesium 137.
In order to now explain a typical functioning of the systems of the prior art of the aforementioned type, reference will be made to an example in the field of gamma spectrometry.
Nevertheless of course, those skilled in the art will easily be able to extend this example to other categories of radiation, including those mentioned above for example.
FIG. 2 shows a signal that can ideally be observed immediately at the output of a gamma photon detector.
This signal includes a multiplicity of pulses of different amplitude and duration which, for example, represent a current generated by the detector by the passage of a photon in the latter.
It will be observed here that this multiplicity of pulses could also represent a voltage generated by the detector.
In any event, in the remainder of this article, a detector current signal will refer to a signal supplied by the detector.
Returning briefly to the pulses, their width, corresponding to a certain duration in time, is a function of a charge collection time.
As mentioned previously, the detector current signal represented in FIG. 2 is ideal.
As a consequence, such a signal is never observable.
In reality, we generally install a preamplifier at the output of the detector in order to effect a first shaping of the detector current signal.
Generally, two cases of preamplifiers are found in the existing systems, namely capacitive negative-feedback preamplifiers and resistive negative-feedback preamplifiers.
By way of information, FIGS. 3 and 4 illustrate a detector 1 followed respectively by a capacitive negative-feedback preamplifier 2 and a resistive negative-feedback preamplifier 3 which includes a feedback loop between an output and an input of an amplifier 4 composed of a capacitance 5 in parallel with a resistance 6.
These preamplifiers are generally followed by a differentiator circuit 7 in the case of capacitive negative feedback and by a pole-zero correction circuit PZ 8 in the case of resistive negative feedback.
The two FIGS. 5 and 6 respectively show an example of an ideal temporal signal at the output of the aforementioned two types of preamplifier when they are driven at the input by a given detector current signal, on the understanding that noise of an electronic nature is not represented here.
Several stages follow the preamplification stage.
Their order and their implementation can vary significantly.
A known and important stage consists of a shaping of the pulses in order to extract information of the energy type from them.
Such a stage is currently referred to as an “energy shaping” stage.
By way of a non-limiting example, it is possible to extract the information from a measurement of the amplitude or the area of each pulse.
In fact it is assumed that these magnitudes are generally proportional to an energy level.
The choice of the type of “energy shaping” has been the subject of considerable research in recent years in the field of the invention, since this difficult stage necessitates many compromises.
For example, in the case of extracting information from the amplitude of the pulses, a compromise requires that there should exist an optimum between:                the very precise acquisition of this information, meaning a high resolution,        the number of pulses per unit of time present in the detector current signal, and        the fraction of these that one wishes to keep in the spectrum, due for example to a phenomenon that is known in itself and which is normally referred to in this field as the “stacking” of pulses.        
For further details on this, the reader can refer in particular to the notions of “output count rate” or OCR, and “input count rate” or ICR.
In order to obtain an optimal solution to this compromise, research ([1], [2]) has established that the use of a filter of trapezoidal shape could constitute an optimum solution in the absence of stacks of pulses and for a certain nature of noise.
Since then, other solutions, still based on the use of this type of filter, have been proposed in order to improve the performance of these systems.
For example, many options for the creation of this type of filter have been proposed [1-8], such as of the digital or analogue or even mixed type, and other methods.
We are also familiar with solutions whose purpose is to improve performance by other means, such as optimisation of the said PZ correction [6,9,10,11], optimisation of a conventional operation that consists of correcting a base line [12], rejection of stacked pulses by correlation between the length and the amplitude of an unstacked pulse [13], and so on.
However, these solutions still involve the use of an optimal trapezoidal filter for the “energy shaping” stage.
Thus, the methods and systems proposed up to the present in any event include a trapezoidal filter and more generally an “energy shaping” stage.
Even though they have rendered great service, the performance of these systems or methods are nevertheless still limited.
In particular, the use of the said trapezoidal filter is still excessively degrading the detector current signal—or at least the preamplified signal—in particular by a temporal lengthening of the pulses.
As a consequence, when the frequency of occurrence of pulses increases (due to an increase in the frequency of the events, such as in the emission of particles), the systems or methods of this type begin to malfunction, ending for example with spectra of low resolution or even misshapen spectra.
This is typically the case toward 100,000 to 200,000 beats per second (counting rate).
And for the systems with highest performance, it is possible to reach 300,000 to 400,000 beats per second.
Another drawback of the aforementioned systems is that their use is still very inflexible.
In particular, it is difficult or even impossible to adapt or modify the parameters of the energy-shaping stage during an analysis of the radiation source.
In an initial phase, it is therefore necessary to ascertain, or at least to estimate as well as possible, the number of pulses per second in order to set up the system in advance, and particularly at the detection level.
Also, if the number of pulses is over-assessed, an excessively short convolution will be chosen, and this will degrade the resolution.
While if this number is under-assessed, an excessively long convolution will be chosen, which will lead to the rejection of many pulses and to distortion of the spectrum.
By way of an example, the systems that, because of their situation, can be set up only when brought into service, are generally set up on a worst case basis.
In particular, setting up on a worst-case basis can consist of setting up the system as a function of the highest measured pulse intensity.
However since this intensity varies over time, the optimal set-up to get the system to work is no longer valid.
Yet another drawback is that the aforementioned systems and methods generally prove to be rather ineffective for detecting pulses of low energy (or amplitude), in particular when they are on the noise limit, or even buried in the latter.
Thus, the performance of these systems and of these methods are very quickly limited when the signal-to-noise (SNR) ratios are low.